Fill level limit switches of this type are applied in many branches of industry, especially in the chemical and food industries. They serve for detecting a limit level and are e.g. used as overfill guards or protection against pumps running empty.
DE-A 44 19 617 describes a device for establishing and/or monitoring a predetermined fill level in a container. The device includes: a mechanical oscillatory structure placed at the height of the predetermined fill level, an electromechanical transducer, which in operation excites the oscillatory structure to oscillate with resonance oscillations, and a receiver- and evaluation-unit, which uses a resonance frequency of the resonance oscillation to determine whether the predetermined fill level has been reached, or not.
The electromechanical transducer has at least one transmitter, at which an electrical transmitted signal is applied and which excites the mechanical oscillatory structure to oscillate. A receiver is provided, which picks up the mechanical oscillations of the oscillatory structure and transforms such into an electrical received signal. The evaluation unit obtains the received signal and compares its frequency with a reference frequency. It produces an output signal, which indicates that the mechanical oscillatory structure is covered by a fill material, if the frequency has a value smaller than a reference frequency, and that it is not covered, if the value is larger. A control circuit is provided, which regulates a phase difference between the electrical transmitted signal and the electrical received signal to a determined, constant value, at which the oscillatory structure executes oscillations with a resonance frequency.
The control circuit is e.g. formed such that the received signal is amplified and fed back to the transmitted signal by way of a phase shifter.
The oscillatory structures often utilize a membrane, on which is arranged at least one oscillation bar protruding into the container. In operation, the oscillation bars are caused to oscillate with resonance oscillations perpendicular to their longitudinal axes. In order that as large a change as possible will occur in the oscillation frequency upon immersion of the oscillatory structure into the liquid, the oscillation bars are constructed to have paddle-shapes on their ends.
When such a device is placed in a container, in which a liquid is present with waves on its surface caused e.g. by a filling process, it can happen that the reference frequency will repeatedly be first exceeded and then gone under, because of the wave motion. The device then is no longer stable upon attainment of the predetermined fill level, but, instead, changes its switch state in short intervals, without the fill level actually making any net change.
This problem is especially noticeable, when the device is installed in a container from below. In such case, when the fill level rises starting from the container floor, the predetermined frequency change at which the device changes its state lies very much closer to the free end of the oscillation bar than when the oscillation bars extend into the container from above. At the free end of the oscillation bar, the resonance frequency of conventional paddle shapes changes very much more rapidly than e.g. in a middle region of the paddle. As a result, small fill level changes in this case cause large frequency changes. This makes the danger of single or frequent switch state changes being triggered by surface waves especially large.
This can be problematic, if the device is part of a large plant, and the fill level information is being used for regulation or control of other units in the plant.